This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Any natural waterway has the ability to assimilate organic matter. When the loading of organic matter exceeds this assimilative capacity, the water resource is impaired for this reason. Waste, whether human or industrial, is treated for safe release into the environment. For example, wastewater from municipalities and industry is treated before discharge into waterways such as rivers. In many cases, these treatments accelerate the natural assimilation process by introducing additional oxygen to the biological process of degrading the waste.
Pollution, or contamination, of water is a serious problem throughout the world, particularly in the United States. Various sources of contamination are responsible for water pollution, including industrial and municipal entities. Industrial entities may discharge liquid or two-phase (liquid/solid) waste indirectly or directly into the environment, such as into rivers and lakes, contaminating the water supply and harming the environment, fish and wildlife. Air pollution is also a problem, particularly industrial air pollution, because airborne contaminants may be collected by rainfall and runoff into bodies of water. Industrial waste may include heavy metals, hydrocarbons, generally toxic materials, and many other known and unknown contaminants. In addition, wastewater and air pollution typically emit an undesirable odor from the contaminants, which may be a result of insufficient wastewater treatment or inefficient industrial systems (e.g., inefficient combustion, chemical reactions or processes, etc.) creating such contaminants.
Municipalities also produce considerable waste. Particularly, combined sewer overflows (CSOs), sanitary sewer overflows (SSOs), and stormwater discharges can create significant problems. Sewage carries bacteria, viruses, protozoa (parasitic organisms), helminths (intestinal worms), and bioaerosols (inhalable molds and fungi) among many other contaminants. Combined sewers are remnants of early sewage systems, which use a common pipe to collect both storm water runoff and sanitary sewage. During periods of rainfall or snowmelt, these combined sewers are designed to overflow directly into nearby streams, rivers, lakes or estuaries. SSOs are discharges of sewage from a separate sanitary sewer collection system, which may overflow prior to reaching a sewage treatment plant. Sanitary sewers may overflow for a variety of reasons, such as inadequate or deteriorating systems, broken or leaky pipes, and/or excessive rain or snowfall infiltrating leaky pipes through the ground. Finally, storm water runoff adds to the problem, as pollutants are collected en route to rivers, streams, lakes, or into combined and sanitary sewers. Storm water picks up contaminants from fertilizers, pesticides, oil and grease from automobiles, exhaust residue, air pollution fallout, bacteria from animals, decayed vegetation, and many other known and unknown contaminants.
Water contamination may be site specific, as with many industrial entities, or it may be non-site specific as with many CSOs, SSOs, and storm water runoffs. Although the discussion has been limited to industrial and municipal waste, contamination may arise from a variety of sources and accumulate in various site specific and non-site specific locations. For example, agricultural waste, pesticides and fertilizers create site specific water contamination, such as in ponds, streams, irrigation, ground water and drinking water for the animals and people.
Today, the most common waste treatment method is aerobic biological degradation, which uses microorganisms, commonly referred to as “bugs,” to biodegrade waste. In a wastewater treatment application, aerobic biological degradation typically involves an aeration/activated sludge process in which oxygen is added to one or more tanks containing the wastewater to be treated. The oxygen supports the microorganisms while they degrade the compounds in the wastewater. To enable the microorganisms to grow and degrade the waste and, ultimately, to reduce the biochemical oxygen demand (BOD), i.e., the amount of oxygen required by microorganisms during stabilization of decomposable organic matter under aerobic conditions, in the treatment system, sufficient oxygen must be available. In some systems, additional oxygen is required to also reduce nitrogen levels in the effluent.
Typically, waste treatment plants use mechanical or diffuse aerators to support the growth of microorganisms. Mechanical aerators typically employ a blade or propeller placed just beneath the surface of a pond, tank, or other reservoir to induce air into the wastewater by mixing. Such mixers generally have relatively low initial capital costs, but often require substantial amounts of energy to operate.
Alternatively, diffused aerators introduce air or oxygen into wastewater by blowing gas bubbles into the reservoir, typically near its bottom. Diffused aerators, depending upon design, may produce either coarse or fine bubbles. Coarse bubbles are produced through a diffuser with larger holes and typically range in size from 4 to 6 mm in diameter or larger. Fine bubbles, on the other hand, are produced through a diffusers with smaller holes and typically range in size from 0.5 to 2 mm in diameter. Diffused aerators typically have lower initial costs, as well as lower operating and maintenance costs, than mechanical aerators.
Mechanical and diffused aerators involve driving off volatile organic compounds (VOC's) and contributing to odor issues while transferring oxygen in a gaseous state into liquid wastewater, with oxygen transfer occurring mainly as a result of diffusion across the gas-liquid boundary. For example, in the case of diffused aerators using pure oxygen, the gas-liquid boundary is defined by the outer surfaces of the air bubbles introduced into the treatment site. Generally, fine bubble aerators are more efficient than coarse bubble and mechanical aerators due to the increased total surface area available for oxygen transfer that is associated with the fine bubbles. The performance of fine bubble aeration degrades over time if regular maintenance is not used.
However, more efficient apparatus and methods for oxygenating wastewater still are needed. Municipal wastewater needs typically grow as the municipality grows in population. To meet increasing needs, municipalities either expand existing wastewater treatment facilities or build additional wastewater treatment facilities. Either option requires additional land and new equipment. Thus, much expense may be saved by enhancing the operating efficiency of existing facilities in response to increased demand for wastewater treatment.
A municipal wastewater treatment process, for example, typically involves a primary treatment process, which generally includes an initial screening and clarification, followed by a biological treatment process, sometimes referred to as a secondary treatment process. The wastewater entering the activated sludge process may have about sixty percent of suspended solids, thirty percent of BOD, and about fifty percent of pathogens removed in the primary treatment (although in some processes primary clarification may be omitted so that the solids otherwise removed are available for food for the microorganisms working in the secondary process).
The activated sludge process typically consists of one or more aeration tanks or basins in which oxygen is added to fuel the microorganisms degrading the organic compounds. After leaving the aeration tank(s) the water enters a secondary clarifier in which the activated sludge/microorganisms settle out. After passing through this activated sludge process the water typically has about 90% of the suspended solids and 80–90% of the BOD removed. The water is ready for either more advanced secondary or tertiary treatments, or for return to a natural waterway. The choice typically depends upon the effluent levels and local regulations.
Alternately, wastewater treatment may occur in a sequencing batch reactor (SBR). SBR treatment generally is the same as an activated sludge system, except that the process is performed in only one tank, whereas activated sludge systems may use several tanks. SBRs may be used as an alternative to an activated sludge process, in regular secondary treatment, or for more advanced treatment processes, e.g., nitrification/denitrification and phosphorus removal. SBRs may process numerous batches per day. Typically, for industrial applications SBRs process one to three batches per day, whereas for municipal applications SBRs may process four to eight batches per day.
The operation of an SBR generally includes five separate phases: fill, react, settle, decant, and idle, although there may be alternatives to these SBR phases depending upon the circumstances involved in a particular application. In the fill phase, wastewater enters the reactor tank through a port near the bottom of the basin, after which the inlet valve is closed. Aeration and mixing may begin during the fill. In the react phase, the inlet is closed and aeration and mixing continues or begins. In the settle phase, the remaining solids settle to the bottom of the basin. In the decant phase, fluid is removed from the surface of the basin by a decanter. During this time settled sludge also may be removed. In the idle phase, the reactor awaits a new batch of wastewater, typically with a portion of the biomass remaining in the basin to provide food for the microorganisms in the next batch.
The owners and operators of wastewater treatment plants often search for ways to lower the cost of remaining in compliance with local, state, and/or federal laws regulating such plants. One way of lower operating costs has been to pursue energy conservation measures to achieve lower operating and maintenance costs. One particular target has been the substantial electricity and other energy costs associated with the operation of conventional systems for aerating wastewater. Aeration can account for more than half of municipal wastewater treatment energy consumption. However, despite past focus on improving oxygen delivery systems to deliver higher levels of oxygen into wastewater more efficiently, there remains a need for further improvement, i.e., an apparatus and method for delivering large quantities of oxygen in conjunction with wastewater treatment applications. Furthermore, a flexible waste treatment apparatus and method is needed to adequately address non-site specific water pollution, for example, in stream water pollution resulting from CSOs, SSOs and storm water runoff, and special and/or smaller applications such wastewater and odor control on farms.